Studies of the mass function ( MF ) of open clusters of different ages allow us to probe the efficiency with which brown dwarfs ( BDs ) are evaporated from clusters to populate the field .
Surveys in old clusters ( age \gtrsim 100 Myr ) do not suffer so severely from several problems encountered in young clusters , such as intra-cluster extinction and large uncertainties in BD models .
Here we present the results of a deep photometric survey to study the MF of the old open cluster Praesepe ( age 590 ^ { +150 } _ { -120 } Myr and distance 190 ^ { +6.0 } _ { -5.8 } pc ) , down to a 5 \sigma detection limit at i \sim 25.6 mag ( \sim 40 M _ { Jup } ) .
We identify 62 cluster member candidates , of which 40 are substellar , from comparison with predictions from a dusty atmosphere model .
The MF rises from the substellar boundary until \sim 60 M _ { Jup } and then declines .
This is quite different from the form inferred for other open clusters older than 50 Myr , but seems to be similar to those found in very young open cluster , whose MFs peak at \sim 10 M _ { Jup } .
Either Praesepe really does have a different MF from other clusters or they had similar initial MFs but have differed in their dynamical evolution .
We further have identified six foreground T dwarf candidates towards Praesepe , which require follow-up spectroscopy to confirm their nature .